Turbine blades, especially turbine blades for gas turbines, are exposed to high temperatures during operation, which quickly exceed the limit of material stress. This especially applies to the regions in the vicinity of the flow inlet edge on which the hot process gas flow first of all impinges upon the blade profile of the turbine blade. In order to be able to use turbine blades even at high temperatures it has already been known for a long time to suitably cool turbine blades so that they have a higher resistance to temperature. With turbine blades which have a higher resistance to temperature, higher energy efficiencies in particular can be achieved.
Known types of cooling are inter alia convection cooling, impingement cooling and film cooling. In the case of convection cooling, cooling air is guided through passages inside the blade and the convective effect is utilized in order to dissipate the heat. In the case of impingement cooling, a cooling air flow from inside impinges upon the inner surface of the blade. In this way, a very good cooling effect is made possible at the point of impingement, but which is limited only to the narrow region of the impingement point and the immediate vicinity. This type of cooling is therefore mostly used for cooling the flow inlet edge, which is also referred to as the leading edge, of a turbine blade. In the case of film cooling, cooling air is guided from inside the turbine blade outwards via holes in the turbine blade. This cooling air flows around the turbine blade and forms an insulating layer between the hot process gas and the blade surface. The described types of cooling, depending upon the application case, are suitably combined in order to achieve blade cooling which is as effective as possible.
Such a turbine blade with impingement-cooled inflow edge, which has ribs and turbulators on the inner surface which faces the impingement cooling passage, is known from EP 1 473 439 A2. In this case, impingement cooling holes through which cooling air can be directed onto the ribs which are arranged on the inner surface, are provided in a bridge which connects the suction-side wall to the pressure-side wall.
In addition to the types of cooling which are described above, the use of cooling means, such as turbulators, which in most cases are provided in the form of ribs, is very common. These ribs are arranged inside the cooling passages which are provided for the convection flow and extend inside the turbine blade. The installation of ribs in the cooling passages causes the flow of cooling air in the boundary layers to be separated and swirled. As a result of the disturbance of the flow which is forced in this way, heat transfer can be increased in the case of an existing temperature difference between cooling passage wall and cooling air. As a result of the ribbing, the flow constantly causes new “re-attachment fields” to be formed, in which a significant increase of the local heat transfer coefficient can be achieved. The service life of known ribs is limited as a result of the high operating temperatures, which is especially a consequence of the geometry which forms the basis of known ribs. The thermal stresses which are associated with the known rib-geometries result in internal cracks which can limit the service life of the rib and therefore ultimately also limit the period of operation of the turbine blade.
For cooling the flow inlet edge, i.e. leading edge, of turbine blades, which during operation is thermally very severely stressed in most cases, cooling passages, which extend parallel to and close to the flow inlet edge, are often formed in turbine blades, to which cooling passages cooling air is fed by means of further cooling passages which are formed in the blades. The convective cooling of the flow inlet edge which is realized in this way is supplemented in the case of film-cooled blades mostly by means of impingement cooling of the inner wall of the cooling passage which extends close to the flow inlet edge. In applications in which no film cooling of the turbine blades is undertaken, the convective cooling is intensified by means of turbulators which are arranged on the inner wall of the cooling passage.
When considered overall, both in the case of film-cooled blades and in the case of blades which are not film-cooled, there is currently still a clear need for improvement with regard to the cooling, especially with regard to the cooling of the flow inlet edge. In particular, the current cooling solutions make no allowance either for an inhomogeneous temperature distribution which develops during the use of turbine blades.